1,721,020 research outputs found
Direct, Regioselective, and Chemoselective Preparation of Novel Boronated Tryptophans by Friedel–Crafts Alkylation
Full text linkA facile synthetic approach to the direct preparation of various novel unnatural boronated protected tryptophans using a regio- and chemoselective electrophilic substitution of 4- and 5-boronated indoles with N-protected dehydroalanine is described. The gram-scale synthesis of two free tryptophan boronic acids is also reported
A simple, modular synthesis of C4-substituted tryptophan derivatives
Full text linkThe modular and versatile synthesis of C4-substituted tryptophan derivatives by direct functionalization of easily available N-acetyl 4-boronate tryptophan methyl ester via transition metal-catalyzed and metal-mediated cross coupling reactions is described. The versatility of the chemistry is highlighted by the gram-scale synthesis of 4-boronated N-acetyl-tryptophan methyl ester and the rapid synthesis of C4-aryl, C4-alkyl, C4-cyano, C4-trifluoromethyl, C4-azido, and C4-hydroxy tryptophan derivatives. The utility of our methodology is illustrated through the quick approach to the tricyclic azepino indole skeleton embedded in many natural product
Observations concerning the synthesis of tryptamine homologues and branched tryptamine derivatives via the borrowing hydrogen process: synthesis of psilocin, bufotenin, and serotonin
Full text linkObservations concerning the synthesis of substituted tryptamine derivatives starting from indoles and 1,n-amino alcohols via the borrowing hydrogen process are discussed. This catalytic, single-step, and modular approach to tryptamines and homotryptamines allows the synthesis of branched and non branched tryptamines as well as tryptamine-based natural products such as psilocin, bufotenin, and serotonin
Concise and Convergent Enantioselective Total Syntheses of (+)- and (-)-Fumimycin
No full textThe concise and convergent total syntheses of (+)- and (-)-Fumimycin have been achieved by taking advantage of strategies for the asymmetric aza-Friedel-Crafts reaction of a highly substituted hydroquinone and N-fumaryl ketimine generated from the corresponding dehydroalanine. The enantiomerically pure natural product and its enantiomer were prepared in seven steps and 22% overall yield by employing both enantiomers of a BINOL-derived chiral phosphoric acid (CPA) catalyst
Asymmetric Alkylation of Cyclic Ketones with Dehydroalanine via H-Bond-Directing Enamine Catalysis: Straightforward Access to Enantiopure Unnatural α-Amino Acids
Full text linkThe growing importance of structurally diverse and functionalized enantiomerically pure unnatural amino acids in the design of drugs, including peptides, has stimulated the development of new synthetic methods. This study reports the challenging direct asymmetric alkylation of cyclic ketones with dehydroalanine derivatives via a conjugate addition reaction for the synthesis of enantiopure ketone-based α-unnatural amino acids. The key to success was the design of a bifunctional primary amine-thiourea catalyst that combines H-bond-directing activation and enamine catalysis. The simultaneous dual activation of the two relatively unreactive partners, confirmed by mass spectrometry studies, results in high reactivity while securing high levels of stereocontrol. A broad substrate scope is accompanied by versatile downstream chemical modifications. The mild reaction conditions and consistently excellent enantioselectivities (>95 % ee in most cases) render this protocol highly practical for the rapid construction of valuable noncanonical enantiopure α-amino-acid building blocks
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
On technical trading and social media indicators for cryptocurrency price classification through deep learning
Full text linkPredicting the prices of cryptocurrencies is a notoriously challenging task due to high volatility and new mechanisms characterising the crypto markets. In this work, we focus on the two major cryptocurrencies for market capitalisation at the time of the study, Ethereum and Bitcoin, for the period 2017–2020. We present a comprehensive analysis of the predictability of price movements comparing four different deep learning algorithms (Multi Layers Perceptron (MLP), Convolutional Neural Network (CNN), Long Short Term Memory (LSTM) neural network and Attention Long Short Term Memory (ALSTM)). We use three classes of features, considering a combination of technical (e.g. opening and closing prices), trading (e.g. moving averages) and social (e.g. users’ sentiment) indicators as input to our classification algorithm. We compare a restricted model composed of technical indicators only, and an unrestricted model including technical, trading and social media indicators. We found an increase in accuracy for the daily classification task from a range of 51%–55% for the restricted model to 67%–84% for the unrestricted one. This study demonstrates that including both trading and social media indicators yields a significant improvement in the prediction and accuracy consistently across all algorithms
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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